Comparative analysis of solar thermal power generation and photovoltaic power generation

Traditional thermal power generation converts energy stored in fossils into heat by combustion and then into electricity. Solar thermal power generation uses a large number of mirrors to focus the direct light of the sun, convert it into heat by heating water or other working medium, and then use the same process as the traditional thermodynamic cycle to form a high temperature. The high-pressure steam promotes the work of the steam turbine generator set, and finally converts the thermal energy into electrical energy. The principle of the typical solar thermal power generation thermal cycle system is shown in the figure.

Schematic diagram of solar thermal power generation thermodynamic cycle system

It is through such links that solar photothermal power generation technology and traditional thermal power generation technology are smoothly integrated. As the thermal power generation technology has already been very mature, it reduces the risk of the overall technical development of solar photothermal power generation.

According to the Special Research Report on China's Solar Energy Thermal Utilization Market and Investment Strategy Consultation Report 2013-2018 issued by China Industry Information Network, solar power generation technology mainly includes solar photovoltaic power generation and solar photothermal power generation. The principle of photovoltaic power generation is that when solar light shines on solar cells, the cells absorb light energy and produce photovoltaic effect. Abnormal charge accumulation occurs at both ends. If the electrodes are drawn out and loaded, the power output will be obtained. Photovoltaic power generation is the mainstream technology in the solar power industry, which is relatively mature. The state has made it clear that the price of electricity on the grid (varies in the range of 0.9-1 yuan/degree in different regions) and the cost of power generation has also dropped to about 0.7 yuan/degree. Photothermal power generation has a relatively short development time in China, and has taken advantage of the integration and control of solar concentrating methods and equipment, high temperature heat transfer and storage, power plant design, etc. Substantive progress has been made, but the commercial performance is relatively small, the power tariff policy has not yet been implemented, and the cost of generating electricity is also high, about 0.9 yuan/degree. However, compared with photovoltaic power generation, solar photothermal power generation has the following advantages:

1) The output power of solar photothermal power generation is stable, the power is adjustable and easy to connect to the grid.

At present, solar photothermal power generation system can improve output characteristics by adding heat storage units or by supplementary combustion or combined operation with conventional thermal power. However, photovoltaic power generation is greatly affected by the intensity of sunlight illumination, which brings great pressure to the power grid after being connected to the grid. Its unique form of power generation makes it difficult to merge with traditional power plants.

Improving the output characteristics of photothermal power generation by thermal storage (trough and tower photothermal power generation). Storing excess heat in the daytime and releasing energy in the evening can realize continuous power supply of photothermal power generation, ensure current stability, and avoid the problem of peak shaving which is difficult to solve for photovoltaic power generation and wind power generation. According to different heat storage modes, the utilization hours and generation capacity of the power station can be increased to varying degrees, and the regulation performance of the power station can be improved.

Improving the output characteristics of photothermal power generation through supplementary combustion or combined operation with conventional thermal power. Solar thermal power plants can use fossil fuels to refuel or run in conjunction with conventional thermal power, so that they can continue to generate electricity in the evening or in continuous cloudy days, and even can steadily bear the base load operation, so that the annual utilization of power generation can be about 7000 hours.

2) Pollution-free solar photothermal power generation

Photothermal power generation is a clean production process, which basically uses physical means to convert photoelectric energy. It has little harm to the environment. The CO2 emission of solar photothermal power station in its whole life cycle is only 13-19 g/kWh. The fatal weakness of photovoltaic power generation technology is that solar cells have a large environmental loss in the production process, which is a process of high energy consumption and high pollution. Industry experts believe that the energy saved by solar cells in the life cycle is not economical compared with the resources consumed by the production of solar cells themselves.

photovoltaic

Photothermal

Power generation principle

Photoelectrons are formed using visible light in sunlight. The process of generating electricity is achieved by using a semiconductor to adsorb and form an electric current.

The use of thermal energy in sunlight is converted into kinetic energy, and the process of generating electricity is further converted into electrical energy using a steam turbine.

Solar energy resources available

60%

30%

Power generation cost

0.7 yuan / kWh

0.9 yuan / kWh

On-grid price

0.9~1 yuan/degree

no

Energy storage system

Use battery for energy storage, short life and high loss

Thermal storage through materials such as molten salt, water, etc., long service life and low loss

Annual electricity generation hours (hours)

1800~2200

Energy storage: 5000 does not store energy: 2000 up and down

Merged with traditional power plants

Can not

Can

Output power characteristics

Unchangeable

Can be changed, adjusted

Cleanliness of the production process

High pollution

Clean

Conversion efficiency

10~20%

15~30%

Floor area (m2/MW)

25~30

35~40

Scope of application

Suitable for small-scale, distributed generation

Due to its commonality with thermal power generation, it is also suitable for centralized large-scale power generation.

Global skill level

Mature technology application

Technology is relatively mature

Global industrialization level

High degree of industrialization

Industrialization is initially formed

Domestic industrialization level

High degree of industrialization

No industrialization

Advantage

Technology and industry are relatively mature

The heat storage cost is low and the efficiency is high. The annual power generation hours are long and can be effectively matched with other power generation. It is the most suitable condition to gradually replace thermal power and serve as the basic energy load.

Disadvantage

There is pollution in the production process, and the stability is not high

High geographical requirements

Comparison of Photovoltaic and Photothermal Power GenerationAccording to different ways of concentrating, CSP technology is mainly divided into: tower, trough, dish and linear Fresnel. The commonality is to use different technologies to heat the working fluid, and then drive the steam turbine to generate electricity. It can also use the Stirling engine in the process of converting thermal energy into electrical energy. Both trough and tower photothermal power plants have now achieved large-scale commercial operation, while the dish and linear Fresnel are in the system demonstration stage. The three types of CSP systems currently widely used are compared in the table below.

Comparison of three CSP systems

Trough

Dish

Tower

Power generation scale (MW)

30~150

1~50

30~400

Operating temperature (°C)

320~400

750

230~1200

System average efficiency (%)

15

25~30

20~35

Commercial status

Commercialized

Complete the demonstration phase

Commercialized

The maximum capacity of the built single machine

280MW

100KW

133MW

Technical risk

low

high

medium

Energy storage

can

battery

Can, such as molten salt

Multi-fuel design

can

can

can

Cost ($/W)

4.0~2.7

12.6~1.3

4.4~2.5

Cost ($/W) does not consider thermal storage

4.0~1.3

12.6~1.1

2.4~0.9

Land occupation

Big

small

medium

Application

Can be connected to the grid for power generation, heating in the middle temperature section and high temperature section

Small-capacity distributed power generation, independent system power supply in remote areas

Can be connected to the grid for power generation, high temperature section heating

Disadvantage

The use of oil as a heat transfer medium limits the operating temperature up to 400 ° C and produces only moderate quality steam

Reliability needs to be strengthened, and large-scale production cost targets are not yet met.

Performance, initial investment, and commercialization are not enough

Source: China Industry Information Network

1. Trough solar thermal power generation

The trough solar collector system is shown below

A parabolic reflector is used to focus sunlight on the central focus line on a solar collector. Under the action of the sun tracking system, the sunlight will be continuously concentrated on the collector tube at the focal line position. The heat is continuously conveyed to the high-pressure steam generator by the hot fluid flowing in the collector tube, and the heat is exchanged through the heat exchanger to produce hot steam. If the generated steam is used for power generation or heating, the hot steam is compressed, condensed and refluxed into the hot steam generator after work or heat release, and then heated again to a continuous cycle of hot steam in the closed-loop system. If the produced steam is used in other production processes and consumed, it needs to be supplemented with water. At the same time, the heat medium fluid passing through the heat exchanger will return to the collector field and be heated again. In order to produce steam when solar energy is insufficient, heat storage tanks can be placed in the system to store excess energy and supply the system when solar energy is insufficient, so as to increase the efficiency of solar energy utilization.

2. Dish Solar Thermal Power Generation

Disk type, also known as disk type, its main technical characteristics are the use of disc parabolic concentrator, it is also a point focus collector, its focus ratio can be as high as hundreds to thousands of times, so it can produce very high temperature. Thermoelectric converters, such as Stirling engines or Rankine cycle heat engines, are installed on their receivers to convert heat energy directly into electricity. It can be used in single or multiple parallel connection, suitable for small-scale power generation, so it is more suitable for remote mountainous areas far away from the power grid to carry out distributed off-grid power supply.

3. Fresnel collector power generation

The Fresnel collector system is shown in Fig. 8. In 1990, Australian scientists put forward the concept of compact linear Fresnel reflector concentrator and steam generation system based on the experience of trough and tower. It was designed by FRAUNHOFER in Germany in 2002, and the Fresnel solar concentrator with 5000 m2 was manufactured by Solamendo Company in Belgium. Compared with groove type, the key advantage of this system is that Fresnel's focusing ratio is large, high temperature can be obtained, and the foundation and motor required for each square meter mirror are few. The system uses standard plane mirror instead of surface mirror which needs special processing method, so that all mirrors are close to the ground, reducing wind load and steel usage, thus reducing the cost.

4. Tower solar thermal power generation

As shown in the following figure, the tower system has the characteristics of large scale, short heat transfer distance, low heat loss, high concentration ratio and high temperature. It is one of the lowest generation costs in several photothermal power generation systems. The solar radiation is reflected on the heat absorber at the top of the tower by using the fixed-heliostat field arrays formed by many fixed-heliostats. The heat absorber is heated to produce steam directly or after heat transfer, which drives the steam turbine to drive the generator set, thus converting the solar energy into electricity. The whole system is mainly composed of four basic parts: concentrating system, endothermic system, heat storage system and power generation system.

At present, trough and tower CSP have been commercialized, and the share of Fresnel and dish technology is still very small, and there is no sign of large-scale development in the short term. The world's trough-type CSP systems account for the highest proportion, because the initial investment in tower-type CSP systems is high and the energy storage of dish-type power generation systems is difficult. However, the tower-type CSP system has high comprehensive efficiency and is very suitable for large-scale and large-capacity commercial applications. In the planned construction of the CSP project, the proportion of towers has exceeded the trough technology. We believe that the future tower-type CSP technology will be the main technical genre of CSP.

Installed ratio statistics of four CSP technologies (as of March 2013)

category

Trough

Tower

Fresnel

Dish

Built

26.9%

0.8%

0.6%

0.04%

Under construction

28.6%

7.4%

1.8%

—

Planned construction

15.9%

17.6%

0.3%

—

Total

71.4%

25.9%

2.7%

—

At present, the trough concentrating system, which is widely commercialized, is mostly tubular truss type, which consumes a lot of steel, is self-important, and has high cost. In addition, it is single-axis tracking, with low concentration ratio and low efficiency of photothermal conversion. The dish concentrating system has high optical efficiency and low tracking error, and adopts a high-efficiency collector, so that it has high thermoelectric conversion efficiency and the like. However, when the concentrating area is increased, the design difficulty of the support frame body is increased, and the amount of steel used is greatly increased due to the requirement of the wind resistance level, resulting in an increase in system cost. As a linear Fresnel concentrating system, although the construction and maintenance costs are relatively low, but the occlusion and shadow problems are widespread, resulting in low system efficiency, and the concentrating multiple is only tens of times. The problem of energy loss on the heat pipe is also a problem that cannot be ignored on linear Fresnel collectors.

Compared with the above-mentioned shortcomings, the composite disc of Qinhuangdao Yuyang Solar Energy Technology Co., Ltd. has the following advantages as a new type of concentrating system:

(1) Focus fixed design (ie, system heat collecting device fixed) - has the characteristics that the tower focus does not move. The invention reduces the design difficulty of the receiver of the solar concentrating system, and solves the problem that the pipeline installation is ubiquitous in other systems.

(5) Application of graphite column for heat collecting core components——Using a graphite column with good thermal conductivity, corrosion resistance and chemical stability, the collector is integrated with the heat conduction energy storage device in a straight-through manner.

Jiangsu SunPower Solar Technology CO, Ltd . is a high-tech enterprise specializing in research and development of solar thermal products. In 2014, relying on Qinhuangdao Yuyang Solar Energy Technology Co., Ltd. to complete the research and success of solar tracking servo organization, and formally form a strategic alliance.

The main products of Jiangsu SunPower Solar Technology CO, Ltd are dish-type and trough-type automatic tracking solar thermal system.Jiangsu SunPower Solar Technology CO, Ltd can provide solution design and implementation services for solar thermal cluster field system, as well as system design and implementation services for medium and high temperature photothermal utilization projects, such as heating, cooling, drying, fermentation and desalination, and completed cooperation with other units. Several pilot projects for CSP.

Jiangsu SunPower Solar Technology CO, Ltd .'s solar thermal system and equipment can be applied in a wide market space. In addition to power generation, it also involves agricultural, industrial auxiliary, solar building integration, desalination and other applications.